Solid oxide fuel cells (SOFCs) are electrochemical devices that can convert chemical energy into electrical energy with very high efficiency. SOFCs also have several other advantages over combustion-based technologies, such as fuel flexibility (H2, hydrocarbon-based fuels such as CH4, CO, etc.), low emission of pollutants (SOx and NOx), and serving to capture CO2 from the anode exhaust stream in high purity form, already separated from N2.
A typical SOFC consists of a dense electrolyte and two porous electrodes, the anode and the cathode. As part of the efforts to develop new energy conversion systems, there is great interest in reversible fuel cells, particularly reversible solid oxide fuel cells (RSOFCs). RSOFCs are single-unit, all-solid-state, electrochemical devices that can operate in both the fuel cell (SOFC) and electrolysis (SOEC) mode, thus acting as flexible energy conversion and storage systems, particularly to store intermittent renewable energy, such as wind or solar. In the SOFC mode, various fuels, such as H2, natural gas, hydrocarbons or syngas, are converted spontaneously with oxygen (e.g., air) at the cathode to electricity and heat. However, when excess electricity is available, the device can be run in the SOEC mode to convert the electrical energy back to chemical energy by the electrolysis of various feedstocks, such as H2O, CO2 or CO2+H2O to fuel.
The most common degradation and cell failure issue for RSOFCs arises at the oxygen electrode when the cell is operating in the electrolysis mode (oxygen evolution at the oxygen/air electrode). This is due to delamination of the electrocatalytic material from the electrolyte. The delamination mechanism is not fully understood, but several processes have been postulated, including high oxygen pressure development, morphological changes in air electrodes, and electrolyte grain boundary separation [1-5]. At the fuel electrode, key problems are coking, sulphur poisoning and morphological changes leading to performance loss.
Therefore, there is a need in the art for the development of a mixed conducting oxide (MIEC) that can withstand electrolysis conditions without delamination, while also exhibiting superior oxygen evolution and reduction activities. There is further need in the art for mixed conducting oxides for use at the fuel electrode that exhibit resistance to coking, which retain activity in the presence of sulphur (e.g., H2S) and which exhibit good retention of performance during operation. Additionally, for implementation of RSOFCs, there is a need for MIEC which function efficiently in both the fuel cell (SOFC) and electrolysis (SOEC) mode.
To date, the most common materials used in RSOFCs are essentially the same as those used for SOFC, namely yttria stabilized zirconia (YSZ) as the electrolyte, a Ni—YSZ cermet as the fuel electrode, and a La1-x SrxMnO3 (LSM)-YSZ composite as the air electrode. The search for higher performance electrode and electrolyte materials for RSOFCs has been a focus of research in recent years, with a particular emphasis on the development of new air and fuel electrodes. At the air electrode, this has included the development of mixed ionic-electronic conductors (MIECs), such as Fe-based perovskites e.g., SrFeO3-δ, and the use of a variety of cation dopants in both the A and B-sites [6-9]. As an example, LaCrO3 and its doped variants are good candidates for application ascathode materials in SOFCs [10]. Other high performance air electrode materials include La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), which has exhibited a low polarization resistance (Rp) of 0.18 Ωcm2 at 800° C. [11], La0.6Sr0.4Fe0.8Cu0.2O3-δ (LSFCu), which has demonstrated a very low Rp of 0.07 Ωcm2 [12]. An example of a good performing fuel electrode is La0.8Sr0.2Cr0.5Mn0.5 O3 (LSCM) (2), which has exhibited a polarization resistance of 0.3 Ωcm2 in H2 at 800° C. [13].
Recently, Chen et al. [14] have shown very good catalytic activity for both H2/CO oxidation and O2 reduction using the same MIEC material at both electrodes (symmetric electrodes), i.e., La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr), which was used for the first time as an SOFC electrode. The selected stoichiometry of the material was based on increasing the electronic and ionic conductivity of a Fe-based perovskite by heavy A-site substitution of La by Sr. In addition, partial substitution of Fe at the B site by Cr was done to stabilize the orthorhombic perovskite and its associated high level of vacancy disorder [15].
Herein the performance of derivatives of LSFCr containing calcium, i.e., La0.3Ca0.7Fe0.7Cr0.3O3-δ (LCFCr), are examined in SOFC. In particular, the use of these MIECs as the oxygen and fuel electrodes in RSOFC is examined Additionally, the resistance of the electrode materials to sulfur is examined.
Usually, MIECs are synthesized by solid-state reactions, where the process involves multiple heating (≥1200° C.) and regrinding steps to help overcome the solid-state diffusion barrier [16]. Some of the traditional methods by which MIECs have been prepared include the sol-gel method [6], the EDTA citrate complexing process [12], the auto-ignition process [7], the Pechini method [9], and most commonly, by using combustion methods [14].
The use of microwave (MW) assisted methods in ceramic materials processing has recently become an active area of research, primarily as their properties depend so strongly on the fabrication method employed [46, 69, 70]. MW methods have been shown to enhance the rate of diffusion of ions and atoms in solid-solid reactions by several orders of magnitude, thus shortening reaction times and lowering the reaction temperature [46, 71]. Furthermore, it may be possible to induce interesting changes in particle morphology and sizes using microwave methods (87).
MW-assisted techniques are understood to be environmentally friendly [52] as they require less energy than conventional material processing methods. It is also known that MW sintering of ceramics leads to a more rapid heating rate and a higher efficiency of heating, also resulting in a lower thermal stress gradient due to the local heating of microwaves [53, 72]. The use of MW-assisted processing is relatively new in the domain of SOFCs.
The main features that distinguish microwave synthesis from conventional methods are faster energy transfer rates, i.e., more rapid heating rates, and the selective heating of materials. This leads to a unique temperature distribution within the material when it is heated in a microwave furnace. During conventional heat treatment, energy is transferred to a material through thermal conduction and convection, creating thermal gradients. However, in the case of microwave heating, energy is transferred directly to the material through an interaction of the material at the molecular level with the electromagnetic waves [50]. The most important contribution in microwave heating may be that the dipoles in the material follow the alternating electromagnetic field associated with the microwave, with its rapidly changing electric field (ca. 2.4×109 times per second). The resistance to this movement generates a considerable amount of heat [51, 52], thus leading to more rapid heating rates.
It has been suggested that, the more complex a material is, the more difficult it is to prepare by using microwave-assisted synthesis. In more complex systems, very good diffusion is required to uniformly disperse three or more cations throughout the sample during the synthesis. The usual solution to this problem is to combine microwave irradiation with other methods, such as sol-gel or combustion synthesis, as has been done for the synthesis of complex perovskites, such as La0.8Sr0.2Fe0.5Co0.5O3 or CaCu3Ti4O12 [16, 53].
MW energy was reported to achieve the sintering of stabilized zirconia in a multimode microwave furnace at 2.45 GHz [73, 74] with sintering temperatures reduced by ca. 100° C. compared to conventional sintering methods, and that a finer grain size was obtained [75]. Gadolinium-doped ceria (GDC) powder was reported to be successfully synthesized using hydrothermal-MW methods with a resulting increase in the ionic conductivity of GDC in comparison with what was achieved using conventional ceramic processing methods [76].
The use of microwaves for the sintering of SOFC electrodes has been reported [77-82]. For example, Jiao et al compared microwave sintering and conventional thermal sintering of anode supports, showing that the anode-supported microwave-sintered cell exhibited a higher initial performance and lower polarization than conventional thermally-sintered cells [77, 80, 82].
There is increasing interest in the art in providing improved methods for processing ceramic materials that provide materials exhibiting desired electronic properties and which reduce fabrication costs. Herein the use of microwave processing for generating certain electrode materials as well as for fabricating the anode-electrolyte-cathode of SOFCs is examined.